Differential control



Feb. 12, 1952 s. c. RocKAFELLow DIFFERENTIAL CONTROL 2 SHEETS- SHEET l Filed June 27 1951 INVENTOR. frz/HR 7 C /Poc/mfnmw Feb- 12, 1952 s. c. RocKAFELLow DIFFERENTIAL CONTROL 2 SHEETS-SHEET 2 Filed June 27, 1951 QA/10d) Patented Feb. 12, 1952 UNITED STATES PATENT OFFICE DIFFERENTIAL CONTROL Stuart C. Rockafellow, Farmington, Mich., as-

signor to Robotron Corporation, Detroit, Mich., a corporation of Michigan Application June 27, 1951, Serial No. 233,770

Claims. 1

This invention relates to differential control and relates more particularly to a device for actuating one relay when the voltage rises above a first predetermined value and actuating another relay when said voltage falls below a second predetermined value which may or may not be the same as said iirst predetermined value.

In previously known differential controls, wherein reliance was placed on the use of magnetic relays, the diiference in voltage between that required for closing the relay and that value at which the relay opened wastoo great for a large number of requirements. relay which required 90 volts for energizing and closing might, when energized, remain closed until the voltage fell to below 60 volts. Thus, wherever the range to be controlled was itself less than 30 volts, this type of equipment is of no value.

By utilizing the sensitivity of thyratron tubes and their accurate reaction to voltage Variations, it has been found possible to provide a differential control circuit which is sensitive to variations of less than one volt. Such a circuit provides control functions of the minutest sensitivity required in various kinds of automatic equipment.

For example, one of the uses of such a control is to control the rate of feed into a chopping machine or into a mill. A current transformer is connected to one ler,r of the main drive motor and, under normal load, the voltage from the current transformed may be assumed, for purposes of this example, to be 25 volts. Then if the material is being fed to the chopping ma chine, or the mill, too rapidly, the driving motor will be overloaded and will draw more current. Thus, higher voltage will be required of the current transformer and this will cause the control device to close the high range relay contact, This contact will then, by any convenient means, progressively close a gate until the feed to the machine is decreased sufficiently to lower the current drain until the voltage from the current transformer returns to the 25 volt level. Similarly, if the machine is being underfed, the current to the driving motor will decrease, causing less voltage to be drawn from the current transformer. This decrease in voltage to the control circuit will cause the underload relay contact to close and this will actuate a conventional external device to open the feed gate progressively until the feed has increased sufficiently to allow the driving motor to draw such normal current as to cause 25 volts to come from the current transformer.

For example, ak

isto provide a control circuit sensitive to very small variations in voltage and which will energize one circuit upon a voltage rising above a predetermined value and which will energize another circuit upon said voltage falling below said predetermined value.

A further object of the invention is to provide such a circuit wherein the extreme sensitivity of thyratrons is utilized.

A further object of the invention is to provide such a circuit having such interrelation between the parts to be energized by the said rising and falling above or below a predetermined value that when one circuit is energized the other circuit is positively inoperative.

A further object of the invention is to provide such a circuit which is readily adjustable to provide a substantial range of sensitivity.

A further object of the invention is to provide a circuit, as aforesaid, having no moving parts excepting electro-magnetic relays.

A further object of the invention is to provide a circuit, as aforesaid, which is simple and is accordingly economical to construct.

A further object of the invention is to provide such a circuit which can be constructed of sturdy parts and hence does not require frequent maintenance. f

Other objects and purposes of the invention will be apparent to persons acquainted with apparatus of this general type upon reading of the following description and study of the accompanying drawing.

In the accompanying drawings:

Figure l discloses diagrammatically the circuit of a preferred embodiment of the present invention.

Figure 2 indicates a modification of one portion of the circuit shown in Figure 1.

Figure 3 discloses a further modification of f a portion of the circuit shown in Figure 1.

Figure 4 discloses a still further modification of the circuit shown in Figure 1. I

Figure 5 is a graph schematically showingr the changes in value of the control voltage as a result oi the operation of the circuit illustrated in Figure 4.

Summary The circuit involves a pair of thyratrons, each having an electro-magnetic relay in its anode circuit, which thyratrons and relay circuits are connected in parallel with a source of alternating potential. A control circuit is provided and is energized by the voltage whose variations are to bring about a controllable result. A negative potential responsive in magnitude to said lastnamed voltage, hereinafter termed the control voltage, is combined With a positive potential in the thyratron associated with the underload circuit and the relative values of said positive potential and said negative potential are such that said thyratron is normally blocked. A positive potential responsive in magnitude to said control voltage is similarly combined with a negative potential in the thyratron associated with the overload circuit and their relative values are such that said thyratron is normally blocked. Thus, as the control voltage increases its positive values, it ultimately overbalances the negative voltage associated therewith and the overload thyra-tron is permitted to conduct and close the relay in its anode circuit. As the control voltage decreases its negative values, positive voltage associated therewith eventually predominates andthe thyratron in the underload circuit' is permitted to conduct and actuate the relay in its'anode circuit.

Detailed description Referring now to the drawing, the conductors LI and L2 constitute a source of alternating potential` hereinafter sometimes termed the power potential. A thyratron I is connected at its cathode to the line L2 by a conductor 2 and is connected at its anode through the normally closed contacts 3 to the winding 4 of a relay 5. The other end of the Winding d is connected to the conductor LI.

The thyratron 5 is connected at its cathode hv a conductor to the line L2 and is connected. its anode through the normally closed contacts 8 to one end of the winding 9 of the relay ifi. The other end of said Winding 9 is connected by the conductor I l to the line LI.

The relay 5 comprises the Winding 4 which when energized closes the normally open contacts I5 and opens the normally closed. contacts 8. The relay Ill comprises the Winding 9 which when energized closes the normally open contacts I5 and opens the normally closed. contacts 3.

The circuits comprising the lines LI and L2 and the respective thyratrons and relay windings are sometimes collectively hereinafter termed the relay circuits and, for convenience, the circuit including the relay 5 will sometimes be referred to as the underload circuit and the circuit including the relay Il! will be sometimes referred to as the overload circuit.

In the particular embodiment here shown, the constant voltage above mentioned and the vari able voltage to which the said relays are to be made responsive, are algebraically added by utilizing a pair of control electrodes in the respective thyratrons. Other methods of adding these potentials may, however, be employed Without departing from all of the purposes of the present invention, and one such possible circuit is illustrated in Figure 2 and will be described hereinafter.

In the preferred embodiment illustrated in Figure 1 the said constant voltage, hereinafter for identication sometimes termed the standard voltage is supplied by a capacitor in shunt relationship With a potentiometer. The standard voltage governing the underload circuit is provided by a rectifier 20 Whose anode is connected to the line LI and Whose cathode is connected to one plate of a capacitor 2|. The other plate of said capacitor is connected to line L2.

The potentiometer 22 is connected n shunt relationship with the capacitor 2 l. A conductor 23 connects the slider 22a of said variable resistor 22 with the control electrode 24 of the thyrati'on I.

rEhe standard voltage for the overload circuit is provided by the rectiiicr 25 Whose cathode is connected through to the conductor I l to the LI and whose anode is connected to one side of the capacitor 25. The other side of said capacitor is connected to the line L2. A potentiometer 2?' is connected in shunt relationship with the capacitor 26 and the slider 21a of said potentiometer 2 is connected by the connector 28 to the control electrode 29 of the thyratron 6. Said capacitors 2| and 2 are of sufficiently large size, in accordance with conventional practice, that a relatively steady potential will appear across the potentiometers 22 and 2l and a reasonably constant potential will appear in the conductors 23 and 23. However, said capacitors should not be too large or they will be too slow in changing potential, as described hereinafter, and the apparatus will be insufficiently sensitive. This size selection is important, but it will bereadily selected according to conventional practice within the principles and limits as stated.

While for reasons of convenience and maintenance, it is preferable to derive said standard voltages from the lines LI and L2, it will be appreciated that other sources of constant potential can be substituted in place thereof, such as using battery cells 2Ia and 26h as illustrated in Figure 3. The apparatus associated with said batteries is the same as that shown in Figure l and needs no separate description.

The controlling circuit commences at the terminals 30 and 3l in the primary winding of the transformer 32 and functions in the preferred embodiment to control the potentials on the control electrodes 33 and 34 of the thyratrons E and I, respectively.

The terminals 30 and 3l are connected to a voltage which is a function of the control voltage as hereinafter described. The terminals of the secondary winding 35 of the transformer 32 are connected to the end of the potentiometer 36. One end of said potentiometer 35 is connected by the conductor 3'I to the terminal 33 and thence to the cathodes of the respective thyratrons 5 and I. The slider of said potentiometer 36 is connected by the conductor 38 to the negative side of a rectifier, as a diode, 39 and is also connected by the conductor 4I) to the positive side of a rectifier, as a diode 4I The positive side of the rectifier 39 is connected by the conductor 42 to the control electrode 34 of the thyratron l and it is also connected to one side of the capacitor 43. The other side of said capacitor 43 is connected to the conductor 3T. A resistor 44 is connected between the control electrode 34 and the cathode of the thyratron I which connection also shunts the capacitor 43.

Similarly, the negative side of the rectifier 4I connected by the conductor 45 to the control electrode 33. Said conductor 45 is also connected to one side of the capacitor 46 and the other side of said capacitor is connected to the conductor 3l. The resistor 4l is connected in the usual manner between the control electrode 33 and the cathode of the thyratron 6 which connection also shunts the capacitor 45.

Operation In the normal operation of the device, wherein the control potential is at the predetermined c 1value, the condition of the apparatus is as folows:

An alternating potential originating in lines LI and L2 is placed across the principal electrodes kof both of the thyratrons I and 6. Both of said thyratrons are non-conductive for reasons appearing hereinafter and hence neither of the relays 5 and I0 are energized.

Pulses from said power source passing the rectifier charge the capacitor 2| and cause a suhi stantially steady voltage across the capacitor 2 l. Apositive potential is taken from that capacitor 2| by the conductor 23 and imposed onto the control electrode 24 of the thyratron I. In similar manner pulses of opposite sign from those passing the rectifier 20 pass the rectifier 25 and charge the capacitor 26 to an opposite polarity, which in this illustration will be negative, and provide a substantially steady voltage across said capacitor. Said negative potential is then conducted by the conductor 28 to the control eleotrode 29 of the thyratron 6.

` Alternating pulses from the control voltage are applied to the primary winding of the transformer 32 and act to impose corresponding pulses in the conductor 3T and on the slider of the variable resistor 36. One group of pulses, as the' negative pulses, pass the rectifier 39 and charge the capacitor 43. Thus, a negative potential is applied to the control electrode 34 in the thyratron I which overcomes the positive potential of the control electrode 24 of the thyratron and said tube is blocked. Opposite pulses, as positive pulses, pass through the rectier 4i and charge the capacitor 46. Thus, a positive potential is applied to the control electrode 3.3 of the thyratron E but said potential is of such value that it does not overcome the negative potential above described as placed onto the control electrode 29 of said thyratron and said thyratron also remains non-conductive.

So long as the control voltage remains at the preselected value, the condition above described will be maintained and neither of the relays 5 and I0 will be energized.

When the control potential increases beyond the preselected value the positive pulses applied to the control electrode 33 will progressively increase and presently, depending upon the differential established between the control electrodes 33 and 29, enable said positive potential to overcome the negative potential on the control electrode 29 and the thyratron 6 will become conductive. This will energize the winding 9 of the relay I 0 and the contacts IIS will be closed. Closing of said contacts will then energize any desired external apparatus. At the same time, the normally closed contacts 3 will be opened in order to prevent any possible energization of the relay 5. Said relay I0 will remain energized for so long as the control potential is of value sunlcient to cause the thyratron 6 to be conductive and as soon as it falls below the necessary value, said thyratron again becomes non-conductive and relay I0 is de-energized. Inasrnuch as the thyratron will become conductive and non-conductive in response to control voltage variations through very small ranges, the energization and de-energization of` said relay I will be effected by much smaller voltage variations than would be possible if such voltages were applied directly.

-lfsaid control voltage decreases, the negative pulses appearing on the control electrode 34 will become progressively less and presently, dependcapacitor.

6 ing upon the potential applied to said control electrodes 34 and 24 by the sliders of the potentiometers 36 and 22, the positive potential on the grid of the control electrode 24 becomes predominate and the thyratron I conducts. This energizes the winding 4 of the relay 5 and thereby closes the contacts I5 for energizing desired external apparatus, and at the same time the energizing of the winding 4 opens the normally closed contacts 8 by which the relay I9 is positively prevented from accidental energization.

It will be recognized that inasmuch as the thyratrons I and 6 may be made conductive, or non-conductive, within very narrow limits of potential variation between their two control electrodes, and inasmuch as the relative potentials imposed unto said control electrodes may be very accurately regulated by the sliders of the potentiometers 36, 22, and 21, the energization and de-energization of the respective relays 5 and I0 may be effected Within extremely narrow limits of control voltage variation.

M odifcations One modification of the device is shown in Figure 2 wherein the standard voltages and the control voltages are added algebraically by other circuits and only a single control electrode is used in each of the thyratrons. The broad principles, however, of the invention are not changed.

In this modiiied circuit, the thyratrons Ib and 6b correspond to the thyratrons I and 6 (Figure l) and are connected through relay windings to a source of power potential in the same manner as shown for thyratrons I and 6 in Figure l. These thyratrons, however, are only provided with one control electrode each instead of the two control electrodes appearing in thyratrons l and 6.

The control voltage is applied to the terminals and 3l of the primary Winding of the transformer 32 in the manner as above described in the form shown in Figure l. Said transformer 32, however, has two separate secondary windings a and 35h connected as appearing hereinafter.

A source of constant potential, as a battery 2 I b, is connected by its negative terminal through a conductor to the cathode of the thyratron Ih. The positive terminal of said battery is connected by the conductor 5I to one end of the secondary Winding 35a of the transformer 32. The other end of said transformer is connected to the negative side of the rectier 39, here a diode, and is thence connected through the conductor 52 to the control electrode 53 of the thyratron Ib. A capacitor 43 is connected between the conductors 5I and 52 in the same manner as appearing in Figure l and a resistor i4 is also connected to `shunt said capacitor.

A battery 26h is connected to the conductor 56 so that its positive terminal is connected to the cathode of the thyratron 6h and its negative terminal is connected by the conductor 54 through the secondary winding 35h to the positive terminal of the rectifier 4I. The positive terminal of the rectifier 4I is then connected by the conductor 55 to the control electrode 56 of the thyratron 6b. The capacitor 46 is connected between the conductor 54 and 55 as shown and the resistor 4l is connected to the shunt of said The operation of this modication will be reasonably clear in the light of the description given above for the operation of the form shown in Figure 1. However, a few additional comments respecting the form shown in Figure 2 may assist in making this operation clear. The general organization and operation with respect to the relays is identical to that set forth above with respect to Figure l. The only difference is in the means for algebraically adding the standard voltages' to the control voltages and applying same to cause said thyratrons to be conductive or non-conductive. Referring to Figure 2, it will be understood that the voltage provided by the battery 2lb, being connected in series with the secondary winding 35a, will be algebraically added to the voltage induced in said Winding and the algebraic sum is placed onto the control electrode 53. The values and sign of said voltages are such that the negative pulses passing the rectiier 3:3v normally overcome the positive potential from the battery 2 lb and hold the thyratron Ib blocked Whenever the control voltage is within its proper range in the same manner and with the same results as is above set forth with respect to the thyratron The potential supplied by the battery 28D is similarly algebraically added to the potential induced in the secondary winding 35D by the control potential and the algebraic sum is placed onto the control electrode 56. The magnitudes of the potentials involved are such that the negative potential originating in said battery 2Gb will, whenever the control potential is within its proper limits, overcome the positive potential originating in the secondary winding 35h and hold the thyratron 6b normally blocked in the same manner and with the same results as above set forth in detail with respect to thyratron E. Variations past a predetermined limit in the value of the control voltage will vary the positive or negative pulses appearing in secondary windings 35a and 35h and make conductive one or the other of said thyratrons and bring about the same control functions as above set forth for the circuit shown in Figure 1.

In some uses of the circuit above set forth as a preferable circuit, it has been ound that the time interval between the controlling element, as a material feeding gate, and the element upon which such control is manifested, as the motoi driving a grinding device, is suiiiciently great that the device has a tendency to over-control and hunt from one side to the other side. Accordingly, in such instances, it will be desirable to employ the additional elements disclosed in Figure 4.

In this iigure, which in all respects excepting L those hereinafter specically described is the same as Figure l in both its construction and operation, there are provided capacitors 60 and and means connecting said capacitors in shunt relationship with the windings 4 and 9, respectively, of the relays. There is also provided a further contact 52 for the relay l0 and means connecting saine between the conductor 28 and a secondary slider 65 in the potentiometer 2l, said slider 59 being located on the side of said principal slider 27a which is adjacent the negative side of the capacitor 2B. A small resistance 54 is provided between the point of connection of contact 62 to the conductor 23 and the principal slider 2'la of the potentiometer 21. A capacitor 53 is positioned between the conductor L2 and the conductor 28.

Similarly a pair of contacts 61, opened and closed bythe relay 5, are connected between the conductor 23 and a secondary slider 10 on the potentiometer 22. Said slider 10 is located between the principal slider 22a and the negative side of the capacitor 2| for reasons appearing hereinafter. A small resistor 66 is placed between the point of connection of said contacts B1 with the conductor 23 and the principal slider 22a. A capacitor 68 is connected by its negative side to the conductor L2 and by its positive side to the conductor 23, said last named connection including a resistor 1 I. A point between said resistor and said capacitor 68 is connected to the secondary slider 10.

In describing the operation of these added elements, it will be assumed that the device controls the motor for opening and closing the gate of a materials feeding conveyor. The control voltage is derived from the driving motor for a. machine, as a Crusher, which is performing an operation upon said materials. The device will effect small, limited, stepwise correction of the part under control, as the gate above mentioned, whenever the control voltage varies only a little from the standard voltage, and it will eiiect continuous and unrestricted adjustment of said part subject to control whenever the control voltage varies widely from the standard voltage. Thus, to continue the example, it may be assumed that the standard voltage is volts and the stepwise corrections are to take place within overload or under-load limits of 30 volts. These limits are controlled in their relationship to the standard voltage by the voltage differential between the position of the slider 21a and the slider 69 with respect to overloads and the slider 22a and the slider 1i) with respect to underloads.

Now continuing with the example above indicated, it may be assumed that the positive voltage in the control electrode 33 rises sutliciently, as to volts, to overcome the negative voltage in the control electrode 29 and permit conduction through the thyratron `li. This charges the capacitor 6l, and simultaneously energizes the winding 9 of the relay l0. This energizing of said Winding 9 closes the contacts I6 and starts the closing of the feeding gate in the manner above described in more detail. Simultaneously the energizing of the winding 9 closes the contacts 62. This connects the conductor 28 to the secondary slider 69 and thus increases the negative voltage on the control electrode 2S to minus volts and simultaneously charges the capacitor 63 in this amount. Since the positive voltage in the control electrode 33 is only 110 volts, this closing of the contacts 62 immediately again blocks the thyratron 6. The charging o! the capacitor 6l holds the winding 9 energized for a predetermined period of time, as one second, which will represent the minimum period of time for which the contacts I6 will be held closed. Thus, even though the thyratron 6 was conductive only momentarily, said contacts IB are held closed for a period of one second and corresponding adjustment of the above mentioned feeding gate, or other items to be corrected, is effected. The capacitor 63 holds the greater negative voltage onto the control electrode 29 for a sufficient period of time, namely, until this capacitor is discharged through the small resistor 64 again to the standard voltage so as to prevent the immediate re-energizing of the thyratron if the control voltage has not in they meantime changed materially in the negative direction. As soon, however, as said capacitor '63 is discharged sutilciently to permit the control voltage of a. 110

volts, assuming that it has remained at that point, to againrender the thyratron 6 conductive, the cycle will be repeated. Thus, the contacts I6 will be closed for intervals having a definite duration, as one second each, and they will be prevented from closing at intervals closer together, as one second, than permitted by the capacitor 63.

However, when the control voltage becomes excessive by a large margin, so that its positive pulses are in excess of 130 volts, then the closing of the contacts 62 will not render the thyratron again non-conductive but it will remain conductive and the contacts I6 will remain closed until said control voltage decreases suinciently that its positive pulses are under the amount determined by the principal slider 21a and the secondary slider 69, in this example 130 volts. It will thus be understood that in the event of small overloads, the corrections will be made with respect to certain time limitations which will insure against both momentary but meaningless energization and which will also prevent chattering of the equipment, and in the event of larger overloads, the control voltage will be brought quickly back to a point near the standard voltage and then it will approach the standard voltage stepwise and suiiiciently slowly to allow any time lags between the parts subjected to control and the parts originating control to elapse and thus permit said parts to become equalized.

Similarly in the case of underloads, the parts react within predetermined time limitations, not necessarily the time limitations as provided with respect to overloads, but for large underloads the reaction is continuous until the control voltage again approaches a point close to the standard voltage. In the case of a small underload, the thyratron I is rendered conductive and the winding 4 of the relay 5 is energized. In the same manner as described above this closes the contacts I5 to -bring about whatever result is desired, as opening the gate of the above mentioned feeder. Simultaneously, the contact 61 is closed and the positive voltage on the control i electrode 24 is thereby reduced by an amount controlled by the secondary slider 10, as from 100 volts down to 70 volts. Thus, if the negative pulses on the control electrode 34 have not fallen below minus 70 volts, the thyratron again becomes blocked. The capacitor 60 holds the winding 4 energized long enough to insure that the contacts I5 are held closed for a minimum period of time, as for one second, and then released. The capacitor 68, having been drained during the period the contacts 6'I were closed, now is again lled by current passing through the small resistor 66. This creates a time lag during which the positive potential of the control electrode 24 is materially under 100 volts so that if the negative voltage on the control electrode 34 has not materially changed, the thyratron I remains blocked and it will be prevented from again conducting until the capacitor 68 permits the positive potential on the control electrode 24 again to rise sufficiently to permit conduction at the original level.

Thus, as with the overload side of the circuit, small decreases in the control voltage will bring about closing of the contacts I5 for minimum periods of time which periods of time are separated by minimum intervals, and larger conditions of underload will ybring about a continuous closing 'of the contacts I5 until the control voltage ap- By wayof further example, a photocell responsive to a particular light wave lengthlassociated with a given temperature, or a pyrometer, may provide a predetermined voltage corresponding to a selected over or furnace temperature. Then as the voltage rises or falls according to the rise or fall of temperature in the furnace or oven, the differential control will be actuated to control other means for raising or lowering the oven or furnace temperature as required to return it to its predetermined value. Still further examples may be found in water level control, conveyor take up controls, overspeed and underspeed controls, various kinds of inspection devices, and protective voltage controls in anticorrosion apparatus for pipe-lines or salt water equipment, such as valves and gates for shipping locks.

While certain selected embodiments of the invention are here shown for illustrative purposes, it will'be understood that a variety of other embodiments will be apparent to persons skilled in the art and that such other embodiments will also be included Within the hereinafter appended claimsexcepting as said claims expressly require otherwise.

I claim:

1. A diiierential control circuit responsive to variations in an alternating control voltage, comprising in combination: a source of alternating power potential; a first thyratron and a rst relay winding in the anode circuit of said rst thyratron and means connecting same in series with each other and with said power source a second thyratron and a second relay Winding in the anode circuit of said second thyratron and means connecting sa-me in series with each other and with said power source, the circuit of said second thyratron and said second relay winding being in parallel with the circuit of said rst thyratron and said iirst relay winding; first and second control electrodes in each of said thyratrons; a source of substantially constant positive standard potential and means applying same between the rst control electrode and the cathode in said rst thyratron; a source of substantially constant negative standard potential and means applying same between the istcontrol electrode and the cathode in said second thyratron; a pair of terminals for receiving said alternating control voltage; means responsive to said last-named voltage for applying negative pulses thereof to the second control electrode in said first thyratron and means applying positive pulses thereof to the second control electrode in said second thyratron.

2. In differential control means responsive to an alternating control voltage, the combination: a source of alternating power voltage; a pair of thyratrons and means connecting same in parallel with each other, cathode to cathode, and to said source of alternating power voltage; a pair of relays, the respective windings thereof being in the respective anode circuits of said thyratrons; means biasing a control electrode in one of said thyratrons positively with respect to the cathode of said one thyratron and means biasing a control electrode in the other of said thyratrons negatively with respect to the cathode of said other` thyratron; means receiving negative pulses from the source of said alternating control voltage and imposing same in opposition to the positive bias of said one thyratron and means receiving positive pulses from the source of said alternating control voltage and imposing same in opposition to the negative bias of said other thyratron; said opposed bias and said pulses being of such magnitude that said control electrodes of both of said thyratrons are normally biased negatively with respect to their respective cathodes suicient to block conduction of said thyratrons; whereby when said alterhating control voltage decreases, said one of said thyratrons will conduct and actuate the relay in the anode circuit thereof and when said alternating control voltage increases, the other` of said thyratrons will conduct and actuate the relay in the anode circuit thereof.

3. In differential control means responsive to a governing alternating control voltage, the combination: a source of alternating power voltage; a pair of thyratrons and means connecting same in parallel with each other, cathode to cathode, and to said source of power voltage; a pair of relays, the respective windings thereof being in the respective anode circuits of said thyratrons; means biasing a first control electrode in one of said thyratrons positively with respect to the cathode of said one thyratron and means biasing a rst control electrode in the other of said thyratrons negatively with respect to the cathode of said other thyratron; means receiving negative pulses from the source of said alternating control voltage and imposing same between the second control electrode and the cathode in said one thyratron; means receiving positive pulses from a source of said alternating control voltage and imposing same between a second control electrode and the cathode in said other thyratron; said last two named means, and said biasing means, being of such character and value as normally to impose said opposed bias and said pulses in such magnitude as to eliect a net negative bias between said rst and second control electrodes and the respective cathodes of both or said thyratrons suicient to block conduction of said thyratrons; whereby when said alternating control Voltage decreases, said one of said thyratrons will conduct and actuate the relay in the anode circuit thereof and when said alternating control voltage increases, the other of said thyratrons will conduct and actuate the relay in the anode circuit thereof.

4. In differential control means responsive to an alternating control Voltage, the combination: a source of alternating power voltage; a pair of thyratrons and means connecting same, cathode to cathode, in parallel with each other and to said source of power voltage; a pair of relays, the respective windings thereof being in the respective anode circuits of said thyratrons; means constantly biasing a rst control electrode in one of said thyratrons positively with respect to the cathode of the second thyratron and means constantly biasing a first control electrode in the other of said thyratrons negatively with respect to the cathode of said other thyratron; a pair of conductors energized by said alternating control voltage; a rst rectifier and a rst capacitor and means connecting same in series between said conductors; a second rectifier and a second capacitor and means connecting same in series between said conductors, said rectiers being in opposite polarity with respect to each other and said capacitors being on opposite potential sides of the rectifers to which they are respectively connected and being connected to each other through one of said conductors; a pair of resistors respectively shunting each of said capacitors; a connection from a point between said rst rectifier and the capacitor associated therewith to a, second control electrode in said one thyratron and a connection from a point between said second rectifier and the capacitor associated therewith to the said control electrode in said other thyratron; means connecting a point intermediate said capacitors to the cathode of each of said thyratrons; said voltages and said several means all being of such character that a normal alternating control voltage will eect a net negative bias in each thyratron of sufficient magnitude to block conduction therethrough; whereby when alternating control voltage decreases said one of said thyratrons will conduct and actuate the relay in the anode circuit thereof and when said alternating control voltage increases, the other of said thyratrons will conduct and actua-tev the relay in the anode circuit thereof,

5. In differential control means responsive to an alternating control voltage, the combination: a source of alternating power voltage; a pair of thyratrons and means connecting same cathode to cathode in parallel with each other and to said source of power voltage; a pair of relays, the respective windings thereof being in the re- 'spective anode circuits of said thyratrons; means biasing a first control electrode in one of said thyratrons positively with respect to the cathode of said one thyratron and means biasing a rst control electrode in the other of said thyratrons negatively with respect to the cathode of said other thyratron; a pair of conductors energized by said alternating control voltage; a first rectifier and a first capacitor and means connecting same in series between said conductors; a second rectifier and a second capacitor and means connecting same in series between said conductors, said rectiers being in opposite polarity with respect to each other and said capacitors being on opposite potential sides of the rectifiers to which they are respectively connected and being connected to each other through one of said conductors; a connection from a point between said rst rectifier and the capacitor associated therewith to a second control electrode in said one thyratron and a connection from a point between said second rectifier and the capacitor associated therewith to the said control electrode in said other thyratron; means connecting a point intermediate said capacitors to the cathode of each thyratron said voltages and said several means all being of such character that a normal control voltage will effect a net negative bias in each thyratron of suflicient magnitude to block conduction therethrough; whereby when said alternating control voltage decreases said one of said thyratrons will conduct and actuate the relay in the anode circuit thereof and when said alternating control voltage increases, the other of said thyratrons will conduct and actuate the relay, in the anode circuit thereof.

6. Means as defined in claim l having also a normally closed pair of contacts in the anode circuit of each thyratron, each of said pairs of con- 13 tacts being openable by energization of the relay in the anode circuit of the respectively other thyratron.

7. Means defined in claim 2y wherein the biasing means for said thyratrons includes: a first rectier and a iirst capacitor in series and connected to said alternating power source, and a first resistor shunting said iirst capacitor; a second rectier and a second capacitor in series and connected to said alternating power source and a second resistor shunting said second capacitor; said rectiers being in opposite polarity with respect to each other; a connection from a point between said iirst rectiiier and said iirst capacitor to a control electrode of said one thyratron and a connection from a point between said second rectier and said second capacitor to a control electrode in said other thyratron, and means connecting a point intermediate said capacitors to the cathode of each of said thyratrons.

8. The combination defined in claim 1 wherein said source of positive standard potential comprises a rectiiier and a capacitor and means connecting same in series with each other and to the source of said alternating power potential and a resistor shunting said capacitor, means connecting a point between said capacitor and said rectifier with said rst control electrode of said first thyratron and means for applying said negative standard potential to the second thyran tron including a second capacitor and a second rectier and means connecting same in series with each other and to the source of said aiternating power potential, said second rectifier being connected in opposite polarity with respect to said first rectifier and means connecting a point between said second rectifier and said second capacitor with the rst control electrode of said second thyratron, and means connecting a point intermediate said capacitors to the cathode of each of said thyratrons.

9. Means deiined in claim 1 including also: means actuated by conduction of said first thyratron for decreasing the positive voltage imposed between the iirst control electrode of said thyratron of its cathode by a predetermined amount and a capacitor shunting the winding of said rst relay.

10. The device deiined in claim 1 including also: means actuated by conduction of said first thyratron for decreasing the positive voltage imposed between the iirst control electrode and the cathode of said first thyratron by a predetermined amount; a capacitor shunting the winding of said first relay; connecting same to said first control electrode and to the cathode associated therewith, and also connected to said source of constant standard positive potential, whereby when said first control electrode is energized to said greater negative charge with respect to the cathode of said iirst thyratron it will be held at such a potential for a limited period of time after said source is removed.

11. Means described in claim i including also: means actuable on conduction of said second thyratron for increasing the negative voltage imposed between the first control electrode and the cathode thereof by a predetermined value and a capacitor connected in shunt with the winding of said second relay.

a further capacitor and means f 12. The device deiined in claim 1 including also: means actuated by conduction of said second thyratron for increasing the negative voltage imposed between the rst control electrode of said second thyratron and the cathode thereof by a predetermined amount; a capacitor shunting the winding of said second relay; a further capacitor and means connecting same to said iirst control electrode and to the cathode associated therewith and connected also to a source of constant standard negative potential.

13. Means deiined in claim 1 including also: means actuated by conduction of said first thyratron for decreasing the positive potential imposed f between the iirst control electrode of said first thyratron and its cathode by a predetermined amount.

14. Means described in claim 1 including also: means actuable upon conduction of said second thyratron for increasing the negative potential imposed between the iirst control electrode of said second thyratron and the cathode thereof by a predetermined value.

15. A differential control circuit responsive to variations in a alternating control voltage, comprising in combination: a source of alternating power voltage; a pair of thyratrons and means connecting same in parallel with each other, cathode to cathode, and to said source of alternating power voltage; a pair of loads, each of said loads being in the respective anode circuit of said thyratrons; means biasing positively a control electrode in one of said thyratrons with respect to the cathode of said one thyratron and means biasing negatively a control electrode in the other of said thyratrons with respect to the cathode of said other thyratron; means receiving negative pulses from the source of said alternating control voltage and imposing same in opposition to the positive bias of said one thyratron and means receiving positive pulses from the source of said alternating control voltage and imposing same in opposition to the negative bias of said other thyratron, said biases and said pulses being of such magnitude that said control electrodes of both of said thvratrons are normally biased negatively with respect to their respective cathodes suiiiciently to block conduction of said thyratrons; whereby when said alternating control voltage decreases, said one of said thyratrons will conduct and energize the one of said loads and when said alternating control voltage increases, the other of said thyratrons will conduct and energize the other of said loads.

STUART C. ROCKAFELLOW.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,366,500 Eastin Jan. 2, 1945 2,412,092 Mayle Dec. 3, 1946 2,425,734 Gille Aug. 19, 1947 2,441,568 Finison May 18, 1948 2,442,491 Gieskieng June 1, 1948 2,478,279 Kochenburger Aug. 9, 1949 2,504,955 Atwood Apr. 25, 1950 2.534,801 Siltamaki Dec. 19, 1950 2,534,958 Deming Dec. 19, 1950 

